The study was initiated to investigate the role of viral and cytoskeletal proteins in the maturation and assembly of endogenous, type B retroviruses of murine origin. Intracytoplasmic type A particles, precursors of such viruses, are associated with microtubule organizing centers (MOTC), the mitotic apparatus and microtubules per se. By arresting cells at various stages of mitosis by Colcemid and vincristine sulfate, we could show that the migration of such precursor particles from the cytoplasm to the kinetochores of the chromosomes is a function of the inhibition of microtubule formation at the pericentriolar cytocentrum as well as the inhibition of spindle fiber formation at chromosomes. The association of type A particles with the mitotic apparatus is reversible by removal of the inhibitor and is inversely proportional to the reattachment of spindle fibers to chromosomes. The active participation of microtubule and cytoskeletal proteins in the intracellular transport and maturation of type B viruses was confirmed by immunoelectron microscopic studies of the distribution of anti-tubulin and anti-actin in virus infected cells.